Field of the Invention
The present invention concerns a method and device for monitoring and optionally controlling the microbiological state in process streams based on the measurement of variables, such as dissolved oxygen or rH or both, optionally assisted by soft sensors, based e.g. on Linguistic Equations (LE).
Description of the Related Art
A common problem in many industrial process systems, e.g. in paper and pulp processing systems, is microbial growth, if microbes are not controlled efficiently. Microbiological growth may cause several problems in the systems. The bacteria in a process stream may cause spoilage of the stream or bacteria may attach to system surfaces forming biofilm or slime. For example, biofilm masses that detach from system surfaces can be carried into the pulp waters and formed into the paper sheet thereby weakening the formed paper sheet quality, e.g. by causing it to tear or causing holes in the paper.
Industrial processes, such as the paper or board manufacturing process, often contain process stages where the process streams are kept in tanks for longer periods of time. These process stages are ideal for the development of microbes, whereby there may be an increase in the microbial content of said streams. Other examples of locations ideal for microbiological growth in a process system are dead zones of process systems (e.g. poor mixing zones).
In the industrial process systems, e.g. in manufacturing of paper or board, microbe analyses are presently carried out in the laboratory due to the lack of fast online measuring methods and devices. The methods, e.g. plate count, are time-consuming and it can take up to 2-3 days to get the results of the analysis. Further, as there always exist a delay between the laboratory analysis results and the changes in the process conditions, the result may be an inadequate biocide dosage to the process stream and, thus, a poor paper quality as well as a poor cost-efficiency of the process. The same is true for other processes requiring biocides to maintain a low level of microbes in a process stream.
WO 2008/101089 describes a method of monitoring microbiological activity under aerobic conditions in process streams based on changes on dissolved oxygen concentrations measured using specified process steps. However, the measurement is claimed not to be suitable for process streams of low dissolved oxygen concentration. Further, the process is complex, requiring cleaning of the measuring probes between measurement points during measurement cycles.
Further, there is not available a method for reliably monitoring microbiological state under anaerobic conditions in process streams.
Thus, there is a need for simpler measurement methods to monitor the microbiological state of process streams, in particular on-line, and optionally to control microbiological state of process streams e.g. by controlling the effective amounts of biocide(s) to be added to the processes to maintain an acceptable microbiological state in said streams. In addition, there is a need to be able to monitor the microbiological state also at low dissolved oxygen concentrations in process streams, and in anaerobic conditions.